PT Cruiser Convertible

Designing, Engineering & Making The Drop-Top

Article Post: 4/30/2004

Gary S. Vasilash

Editor-in-Chief, Gardner Business Media, Inc.

PT Cruiser Convertible. Key suppliers include Continental Teves for the ABS control unit; Hella for the headlamps and tail lamps; Valeo for the HVAC and the engine cooling module; Decoma for fascias; TRW for remote keyless entry; Johnson Controls for seats; Lear for door trim, hard trim, instrument panel, overhead system; Intier for the window regulators; Denso for the wiper module and blades; ArvinMeritor for springs; Edscha for the soft top..

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"We had to go from a four-door to a two-door and yet to make it look like the two vehicles were designed as one." That's Brandon Faurote, senior manager, PT Cruiser Design, talking about one of the many challenges that they grappled with in turning out a new version of the distinctive PT Cruiser. Sure, the front ends would be the same. What he's talking about is making the two look harmonious, like they belong together, not as though this was some sort of kluge created with a saw to cut off the top and a couple of sheet metal panels to cover where the rear doors would otherwise be.

Consider a convertible with the top down. In effect, what was "inside" is now "outside." So, Faurote says, an effort was made to connect the two. They created what's called a "sport bar." Top up, it looks like the B-pillar. Top down, it is a body side-colored spoiler—although one with slightly odd positioning, as it's not in the back, where spoilers ordinarily reside. Yes, it is a structural component (it is a hydroformed assembly) that helps make the car stiffer than it would otherwise be. Yes, it does perform an aero function: it helps reduce wind noise so that those who sit in the back seat can talk with the top down without having to yell at one another. But it performs a visual function, too: The top stack (or retracted roof) is so far back that the sport bar serves to balance proportions along the side of the vehicle (i.e., windshield/sport bar/top stack).

Engineering

A fundamental to building a solid convertible is a solid structure. While one of the simple ways to add structure is to add mass, not only is that counterproductive vis-à-vis obtaining good fuel mileage, but there is also the issue of excess weight reducing the nimbleness of the vehicle (just what you don't want is a convertible that's as solid as a rock and just as maneuverable). According to Dennis Krozek, director, Vehicle Development and Program Management, there were specific areas where the CAE-based strain density plots indicated that improvements would be helpful, so reinforcements were applied in these five areas: on the sill between the A and B pillars; cross-car, by the B-pillar; the rear seat structure between the wheelhouse; the beltline, from the B-pillar rearward; inside the two doors (beams were added). The aforementioned sport bar also contributes to rigidity. As a result, the body-in-white has a twist stiffness of 4,693 ft.-lb./deg. and a bend stiffness of 42,800 lb./in. In addition to which, the front strut, rear Watt's linkage, rear shocks, and rear trailing arm mounting structures are stiffened. What's interesting to note is that compared with the sedan, the addition of mass for the convertible is a comparatively mere 150 lb., so it is clear that strength doesn't necessarily require a whole lot of extra weight—if you engineer it right.

Manufacturing

The sedan and the convertible are being produced at the Toluca Assembly Plant (Mexico). Paul Scapini, program manager, PT Cruiser, notes that the workers there have built over 640,000 sedans during the past four years. Since the sedan has been in production, there has been the addition of a new paint shop and the assembly area has been expanded to accommodate the production of the variant version. A new automated underbody line has been installed specifically for the convertible. The trim, chassis and final line is shared by the two vehicles. There is no batch building of PTs. What is built is driven by customer orders. There is a reticence to offer production projections. But it is probably going to be on the order of 25,000 to 35,000 convertibles per year.

One of the manufacturing issues related to making a convertible that doesn't exist in making hardtops is installing the top module. After all, location is everything, especially when an objective is to produce a vehicle with little interior noise. So they've installed an ergonomic assist arm in Toluca to lift and position the top module into a setting fixture. This aligns the top with the rear quarter panels so that it can be attached where required.

To make sure that they were going to be able to build high-quality vehicles from the get-go, a select group of individuals traveled from Toluca to the Chyrsler Auburn Hills pilot plant to work out and refine the process.